Production of high resolution patterned images used in the fabrication of miniaturized circuits depends upon the exposure of selected portions of a radiation curable material, commonly called a resist, to incident radiation which initiates reactions within the exposed portion of the resist causing it to become more or less easily removed with respect to the unexposed portion when developed. Typically, resists are coated onto substrates composed of materials used in the manufacture of large scale integration devices, etc. For polymeric resists, the reactions are generally either crosslinking or chain scission. If the reaction is crosslinking, the polymer material is generally called, following photographic terminology, a negative resist because the crosslinking renders the exposed portion relatively insoluble with respect to the unexposed portion and permits removal of the unexposed portion of the resist during development. After development, the now bared substrate may be modified by removal of substrate material or by deposition of new material. In production of typical devices, for example, semiconductor integrated circuits, the described processing sequence will be repeated several times, i.e., the crosslinked resist is stripped off and after subsequent processing of the substrate layers, a new layer of resist is put down and exposed, etc.
Selected portions of the resist coated substrate may be exposed in any of several ways. For example, selected portions of the resist may be sequentially illuminated by a scanning radiation source. Alternatively, a radiation source that exposes all of a mask covered with a pattern formed by material opaque to the radiation may be used to illuminate selected portions of the resist material. The radiation used may be electrons or electromagnetic radiation in the visible, ultraviolet or x-ray region. X-rays and electrons, having wavelengths shorter than radiation in the visible or ultraviolet regions, afford the promise of higher resolution patterned images. X-ray lithography is one of a number of technologies currently being investigated for the fabrication of very high resolution semiconductor devices. In addition to the promise of high resolution, x-ray lithographic systems are potentially both inexpensive and comparable in simplicity with optical proximity printing systems. The entire wafer can be exposed in one step and resolution is not limited by scattering, diffraction effects, standing waves or reflections from the substrate.
A successful lithographic system depends upon the availability of a resist having both good sensitivity to the incident radiation and good adhesion to the substrate. The former requirement is imposed by the necessity of having rapid throughput and the latter requirement is imposed by processing steps including resist pattern development and substrate etching. The first x-ray resist materials were generally polymers such as poly(methyl methacrylate) and poly(glycidyl methacrylate-co-ethyl acrylate) developed primarily as electron beam resists. The latter material, often referred to as COP, has excellent sensitivity to electron beam radiation and good adhesion properties.
However, these materials generally have a sensitivity to x-ray radiation that is too low for practical x-ray lithographic systems and resists were developed specifically for x-ray lithography. U.S. Pat. No. 4,061,829, issued on Dec. 6, 1977, to G. N. Taylor describes a class of chlorinated or brominated polymeric negative resists for x-ray lithography. Good sensitivity to x-rays is obtained by these resists primarily, it is believed, because chlorine and bromine atoms have a generally high mass absorption coefficient for x-rays and can be incorporated into the polymer in high weight percents. Within the class of poly(chloroalkyl acrylates) described, poly(2,3-dichloro-1-propyl acrylate), commonly referred to as DCPA, was found to exhibit the best overall properties as an x-ray resist. Improvement in both the resolving and adhesive properties of the resist would be particularly desirable for the cases when 1 micron initial and 0.5 micron final thickness films of DCPA are required to obtain better step coverage on integrated circuit wafers than thinner films would provide.
It might be thought possible to improve both adhesion and resolution by simply mixing DCPA with another polymer or x-ray resist having either or both better adhesion and resolution. However, mixtures of two or more polymers generally are not compatible. Phase separation of the polymers occurs when they are not compatible. Compatibility of any polymer mixture used as a resist material is important because the phase-separated regions are likely to lead to defects which can decrease resolution or adhesion of the final patterned film. In the phase separated regions there are, in effect, two resists present and the different radiation sensitivities of the two resists can result in different amounts of crosslinking in exposed areas. After development, there may be undeveloped spots, opaque spots or edge roughness in the pattern.
It is generally difficult, if not impossible, to predict the compatibility of those polymers that do form compatible mixtures. This difficulty becomes more pronounced when one or more of the polymers in the mixture has a high weight average molecular weight (M.sub.w). DCPA, when used as an x-ray resist, has M.sub.w desirably between 300,000 and 3,500,000 g/mole.